No. 12 Microprocessors

What are they?

All personal computers are based on a special type of electronic circuit called the microprocessor. Often termed a "computer on a chip", today's microprocessors are an elaborate arrangement of miniature transistors, called an integrated circuit or IC. Integrated circuits are sometimes called chips because of their construction from a single piece of silicon.

How do they work?

Microprocessors have built-in memory areas called registers. This is where they perform all their data manipulations and calculations. The signals going into the microprocessor are digital pulses, the patterns of which carry the command that tells microprocessor what to do. The entire repertory of these functions and their names is called the command set. The internal wiring of the microprocessor determines what it does in response to the command. In effect, a computer program, called microcode is built into the hardware.

Microprocessors store and manipulate digital bits. They also move data around. The connections they use to move data in and out make up the microprocessor's data bus. To locate the data that is stored in memory outside of the chip itself, microprocessors use an address bus to indicate to the rest of the computer which memory area it needs to access. A bus is an internal pathway along which signals are sent from one part of the computer to another.

Microprocessors differ in the resources they devote to these functions and this effects the speed with which they will execute a command. Microprocessors can have different numbers of registers and the registers can be of different sizes. Registers are measured by the number of bits they can work with at one time. For example, a 16-bit microprocessor has one or more registers that each hold 16 bits of data at a time.

The number of bits in the data bus influences how quickly the microprocessor can move information. The more bits a chip can use at one time, the faster it is. Microprocessors can have 8-, 16-, or 32-bit data buses. In addition, the number of bits available on the address bus controls how much memory a microprocessor can address. For example, a chip with 16 address lines can work with 64 kilobytes of addresses.

Why do I need to know about microprocessors?

Because they operate your computer and influence the price you pay. Each new generation of chip makes the microcomputer more powerful. The first general purpose microprocessor was the Intel 4004, manufactured in 1971. Its registers handled four bits at a time. By 1978, Intel had introduced the 8086, a chip with a full 16-bit data bus structure. Intel introduced the 8088 a year later. This chip had 16-bit registers and 20 address lines. Its data bus, however, was 8 bits, allowing it to truly exploit the readily available 8- bit support hardware. IBM designed its first personal computer around the 8088 chip. Many computers in use in Africa today still use the 8088 chip, the first generation of computer chips for the personal computer. The second generation of computer chips came in 1984 with the 80286 (often referred to simply as the 286) Intel chip. This chip uses a full 16-bit data bus with 16-bit internal registers. It operates five times faster than the 8088. It also has superior memory; with 24 address lines it can address 16 megabytes of memory. This chip is used in IBM's AT personal computer, a model that is in widespread use in Africa today.

What are the currently used microprocesors ? (CD-Rom Editors note : till 1994!, now the Pentium is the most used processor)

Intel's 80386 (often referred to simply as the 386) microprocessor is the first 32-bit processor to be used in personal computers. It manipulates up to 32 bits of data at one time and processes instructions two or three times faster than the 80286. The 80386's instruction set (the bit patterns it recognizes as commands to perform its various functions) is a superset of that of the 80286, so older software will run on the newer chip. But the 80386 has new features also. Protected mode includes instruction sets for managing memory, making it possible for an operating system to safely run several programs at the same time. Virtual 8086 mode, simulates not just one 8086 but an almost unlimited number of them, all at the same time. This mode thus allows a single 80386 microprocessor to divide its memory into many virtual machines, each one acting like it is an entirely separate computer equipped with an 8086 microprocessor. This means you can simultaneously run several DOS programs on one computer. This is called multi-tasking.

There are several types of 80386 microprocessors. Most are distinguished by the speed at which they run; speed is expressed in megahertz. The higher the megahertz number, the faster the processor. There are also DX and SX versions of the 80386. The DX communicates with RAM over a path that is 32 bits wide. The SX handles data internally, 32 bits at a time also, but it communicates with RAM at only 16 bits. The SX is easier and cheaper to incorporate into older PC designs.

Since the 80386 was released, Intel has also created an 80486. It too manipulates data 32 bits at a time but it includes two new features. One is a built-in 8 kilobyte RAM cache (see below) that works similarly to an external RAM cache to ensure that the processor is not forced to wait for the data it needs to do its work. The other is a built-in math co-processor, a set of instructions streamlined for handling complex math. The pentium is the latest microprocessor from Intel. With more than three million transistors, it offers up to twice the processing speed of a 80486 DX. The gains are made possible by a new design that allows the microprocessor to carry out two commands simultaneously.

What is a RAM cache?

Some memory chips are faster than others at refreshing the electrical charges in the capacitors that hold data. The refresh rate -- usually given in nanoseconds or billionths of a second, affects how quickly data can move from memory to the microprocessor. The faster the RAM chips are, the more expensive they are. To keep costs down, some personal computer manufacturers use slower memory chips for most of the PC's memory and a few faster, more expensive RAM chips on the motherboard as an external RAM cache. The cache - - often consisting of 64 to 256 kilobytes of memory -- helps move data between the main memory and processor with the least delay.

Other microprocessors

Motorola chips are used in the Macintosh. The 68000 was used in the entry-level Macintosh Classic. The 68020 uses a full 32-bit architecture and powers the original Macintosh II. The 68030 is faster and includes special features for virtual memory management. Look for this processor when buying a Macintosh today because it includes the circuits needed to take full advantage of System 7 (see Technology Fact Sheet, Number 11). The 68040 is analogous to the Intel 80486DX and powers the high-end Quadra models of Macintosh computers.

What does this mean for Africa?

Many computers in Africa are running on chips that are now considered obsolete. While these may be perfectly satisfactory to run simple programs, one at a time, they are not capable of running the latest software. IBM ATs, especially, are suitable for operating simple bulletin board software. Chip prices are falling fast and because faster processors and more memory are needed to run many of the new software programs, you should be buying new computers with at least 80486 or pentium chips. Anything less than a 80386 is going to have limited applications.